Most fossil fuels, such as crude oil, blue water gas, carbureted water gas, and in particular coal, have varying amounts of sulfur contained therein. When burned in the presence of oxygen, sulfur dioxide is formed. The sulfur dioxide is a noxious gas and, if released into the ambient air, combines with water to form destructive sulfurous acid. As an example, a 650,000 KW power plant burning coal with a 3% sulfur content will generate about 39,000 pounds of sulfur dioxide per hour. To avoid severely polluting the atmosphere, this sulfur dioxide must be removed from the stack or exhaust gases. The sulfur dioxide becomes a particularly troublesome problem, since shortage of crude oil has increased the use of coal having higher percentages of sulfur therein.
One somewhat effective method of removing sulfur dioxide from stack gases is to pass the gas through scrubber wherein the sulfur dioxide is reacted with an inorganic alkaline substance to form a non-gaseous sulfur compound which exits the scrubber with the liquid effluent therefrom. Such scrubbers include spray towers, packed towers, fluid bed reactors, etc.
A wide variety of inorganic alkaline materials have been used to form the scrubbing liquid. Aqueous solutions comprising at least one of the metallic carbonates of calcium, magnesium and sodium have been especially popular. These metallic carbonates react with the sulfur dioxide to form carbon dioxide and metallic sulfite. The metallic sulfite is normally oxidized to a metallic sulfate by the excess oxygen remaining in the stack gases after the fuel burning operations. Functionally, a metallic carbonate, which is highly soluble in the aqueous scrubber solution and which also reacts to form a highly soluble metallic sulfate (sulfite), is preferred for use in the scrubber. The carbonates and sulfates of sodium and especially magnesium have high relative solubility, but are somewhat short in supply and substantially more expensive as compared to the less soluble calcium carbonates and sulfates. Thus, in the past calcium carbonate has been extensively used in scrubbers because of the large supply, relatively lower cost, and because the scrubber effluent can be safely disposed of since the calcium sulfate (gypsum) therein is relatively innocuous to the environment.
Unfortunately calcium carbonates and sulfates require extreme care with regard to scrubber operational parameters, such as temperature and pH control. Slight variations cause heavy scaling of the scrubber and even optimum control conditions often result in some scaling, thereby requiring eventual shut-down and cleaning of the scrubber internal structure. In addition, due to changes in environmental law, expense and public opinion, it is becoming impractical to land-fill large amounts of scrubber effluent sludge material. It is estimated that as much as 18 million tons of scrubber sludge is produced in the United States per year, substantially all of which must be land-filled under present scrubber processes.